RFR: 8237484: Improve module system bootstrap

[Claes Redestad]

Hi,

a few module bootstrap micro-optimizations.

Webrev: http://cr.openjdk.java.net/~redestad/8237484/open.01/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8237484

Testing: tier1-3

/Claes